Tumor Necrosis Factor (TNF) and it homologues play important roles in a wide array of biological processes including the acute phase response, cell growth and apoptosis, inflammation and lymphocyte activation. TNF has two receptors: TNFR1and TNFR2. TNFR2 represent a family of transmembrane proteins with intracellular region, which lack a death domain. Each of TNFR2 family receptors may associate with one or several TRAF signal transduction molecules. TRAF2, TRAF3, TRAF5, and TRAF6 were shown to participate in signal transduction and activation of NF-kappaB and AP-1 transcription factors. Little is known about TRAF1 function except it is predominantly expressed in activated lymphocytes and is associated with a number of distinct members of the TNFR family, including TNFR2, CD27, CD30, 4-1BB, OX-40, HVEM, TRANCE-R, XEDAR, AITR and others. TRAF1 also binds to latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV). To elucidate the function of TRAF 1 we have generated TRAF 1-deficient mice. Compared to wild type T cells, TRAF1-deficient T cells exhibit markedly increased proliferation to anti-CD3 stimulation. Activated T cells from TRAF1-deficient mice, but not from their normal littermates, responded to TNF by proliferation and activation of NF-kappaB and AP-1. Furthermore, skin from TRAF1-deficient mice was hypersensitive to TNF-induced T-cell mediated necrosis. Taken together, these findings suggest that TRAF1 is a negative regulator of TNF signaling. We propose to apply state of the art knowledge and techniques to further understand TRAF1 function in T and B lymphocytes. We plan to address three important questions: 1. What is the precise biochemical mechanism of TRAF1 negative regulation of TNFR2 signaling? We will study the molecular mechanism of inhibition of TNFR2 signaling by TRAF1. We will determine the role of TRAF1 in signal transduction by TNFR2 in the absence of TNFR1 and signal transduction by TNFR1 in the absence of TNFR2. For this purpose we will breed TRAF1-deficient mice with mice deficient in TNFR1 or TNFR2.2. TRAF1 might have similar function for other members of TNFR2 family expressed on T cells, including OX40, 4- 1bb, CD30, and CD27. Then again, TRAF1-/- mice, in opposite to OX40-/- mice, were hyperresponsive in a model of allergic asthma. We propose to determine the role of TRAF1 in OX40 signaling in vitro and in vivo.3. What is the role of TRAF1 in signaling by EBV LMP1? We will analyze the role of TRAF1 in EBV LMP1 signaling and B cell.